Multilayered products are known which comprise layers that contain transparent thermoplastic plastic materials.
Polycarbonate sheets are known in particular. They are produced for a multiplicity of purposes of use. They are produced, for example, by the extrusion of compositions which contain polycarbonate (molding compositions) and optionally by coextrusion with molding compositions which have an increased content of UV absorbers.
Polycarbonate sheets are known from EP-A 0 110 221.
For the long-term protection of polycarbonate sheets from yellowing due to UV light, EP-A 0 320 632 teaches that the sheets can be provided with a coextrusion layer which contains high concentrations of UV absorbers that have low volatility.
EP-B 0 678 376 and EP-B 0 595 413 teach that for sheets made of polyesters, particularly sheets comprising copolyesters formed from aromatic dicarboxylic acids and mixtures of two aliphatic diols, e.g. PETG, protection from weathering is achieved by coextrusion with outer layers which contain high concentrations of UV absorbers, e.g. those based on benzotriazoles.
A panel made of polymethyl methacrylate with a content of light-reflecting particles aligned parallel to the surface is known from German Patent Specification DE-C 25 44 245. Its layer thickness is designed so that visible light is substantially transmitted and infrared radiation is substantially reflected.
This known body contains light-reflecting particles, which are contained in the base material that is made of polymethyl methacrylate. They are incorporated in the liquid methyl methacrylate monomer, and the latter is introduced into a polymerization chamber formed by parallel glass plates and is partially polymerized. Until this point in time, the particles have descended on to the lower glass plate. The particles are aligned parallel to the surface by a parallel displacement of said plate and are held in this position whilst polymerization is continued. Due to this treatment step, this method of production is costly.
EP-A 340 313 describes solar radiation coatings for ships, tanks, buildings and the like, in order to reduce the heating thereof in the sun. These coatings contain a binder and a heat-reflecting pigment, and optionally contain any desired coloring pigments.
According to EP-A 428 937, sheets of polyethylene for greenhouses are provided by brushing or spraying with a coating which contains light-reflecting pigments in a matrix of lacquer binder vehicles. Since the pigment particles are not oriented by the application process, they only exert a shading effect and result in unsatisfactory transmission, and due to the slight extent of adhesion of customary lacquer binder vehicles to polyethylene, the coating can easily be washed off from the coated sheets with water.
EP-A 0 548 822 describes PMMA sheets and polycarbonate sheets which contain special pearlescent pigments in the coextrusion layer. These pearlescent pigments consist of a support material, e.g. mica, which is covered with a layer of titanium dioxide of thickness 60 to 120 nm.
The production of these pearlescent pigments is described in DE-A 196 18 569, for example. EP-A 0 548 822 teaches that a selectivity parameter (SP) greater than 1.15 is necessary in order to achieve sufficient IR reflection. The SP is defined as follows.
The ratio T/g is also termed the selectivity parameter SP (according to DIN 67 507). This ratio is the percentage light transmission in the visible region divided by the percentage transparency to radiant energy. The SP is a measure of the IR reflection and in this respect is also a measure of the efficacy of solar protection glazing; it should therefore be as high as possible.
It is known that the sheets described in EP-A 0 548 822 contain 20 to 40% by weight of a pearlescent pigment in the coextrusion layer in order to achieve the requisite high selectivity parameters. A disadvantage is that it is necessary to use a high content of pearlescent pigment. These sheets are therefore very costly.
DE-A 100 06 651 teaches that pearlescent pigments which comprise three or more layers of titanium dioxide and silica on mica result in a particularly low extent of yellowing due to weathering during the weathering of plastics materials which contain these pigments.
Pearlescent pigments have the disadvantage that they result in a matt surface when they are used in an outer layer of a multilayered product.
Moreover, a high content of pearlescent pigment is necessary.